The thermoforming process is widely used in the packaging industry to make various products from sheets of thermoplastic material. The plastic is fed through the machines, either continuously from a roll or in cut sheets, and is subsequently heated, to allow forming of the plastic, shaped in molds to produce the shapes of the various desired parts, and cooled and trimmed to separate the parts from the surrounding scrap sheet. The remaining portion of the sheet from which the parts have been cut--commonly called a skeleton--is waste material and is discarded or is sold as scrap plastic. In some applications, such as where the plastic sheet is pre-printed or where the sheet comprises a laminate of two or more base sheets of plastic, the skeleton cannot be reclaimed and must be discarded.
Thermoforming equipment typically includes an oven or heating station, in which the sheet material is heated to a softened state, a forming station in which the heated plastic is formed by molds into the desired shapes, and a trim station where the formed parts are cut. The standard apparatus for carrying the material through these various stations includes either endless chains with projecting spikes, or mechanical clamps carried on endless chains. The typical spiked chains are mounted along the lateral edges of the incoming sheet and the spikes engage the sheet and puncture it to provide a mechanical connection between the edges of the sheet and the chains. The spikes carry the sheet forwardly as the chains move and also provide lateral support for the sheet when the sheet is formed in the mold, an operation which tends to pull in the edges of the sheet. In order to insure that the spikes do not pull through the sheet when lateral tension is placed upon it, the spikes are typically spaced one-quarter to one-half inch inwardly from the edges of the sheet. Only the portion of the sheet inwardly of the spiked chains and the guides for the chains can be thermoformed, and the outer edge portions are left as waste plastic after the thermoforming process is completed. The wasted edge portion on each side of the sheet is typically three quarters to one inch wide.
In addition to the relatively large amount of waste material that is generated when using spiked chains to carry the plastic sheet through the thermoforming equipment, other disadvantages of this type of conveyance have also been noted. The physical puncturing of the plastic sheet by the spikes creates a small amount of particulate plastic which can create problems where extreme cleanliness must be observed, such as in medical packaging applications, and clamp type holders or mating clamping chains may have to be used in such situations. Such clamp type holders and clamping chains also cover a substantial portion of the sheet which cannot be thermoformed. The endless chains which carry the spikes or clamps naturally have a certain amount of tolerance in each link which increases with wear; and the accumulation of the tolerances in each link, over the long length of the continuous chain, results in a loss of precise registration of the plastic sheet as it passes through the machine. The chains, because of their inertia, also can tend to overshoot the intended stopping point when the chain drive is halted, so that the formed plastic articles may not precisely coincide with the cutting edges in the trim press. Such inertial effects additionally tend to limit the speed of operation of the machine. Because the chains are subject to mechanical stresses during the repeated stopping and starting motions, and because they are subjected to heating and cooling as they pass through the equipment, they require periodic maintenance and replacement.
An improved manner of conveying plastic sheet through a thermoforming machine so as to minimize the portion of the sheet which is wasted is disclosed in copending application Ser. No. 345,526, filed Feb. 2, 1981, now abandoned. The edges of a plastic sheet to be thermoformed are articulated so as to provide inwardly facing abutment surfaces at the edges of the sheet. The articulated edges of the sheet are then inserted into slots in guide rails which extend through the thermoforming machine. The slot in each guide rail is slightly larger than the thickness of the sheet, and the slot opens onto a cavity within the guide rail which has engagement surfaces extending away from the slot. The abutment surfaces on the sheet edge mate with the engagement surfaces within the guide rails to prevent the sheet from slipping out, thus providing lateral support and longitudinal guiding of the sheet.
The sheet, while held at its edges by the guiderails, is sequentially heated and softened, advanced to a forming station where mold portions engage and form the softened plastic, and then advanced to a trim station where the formed parts are cut from the remainder of the sheet. It has been found that the plastic sheet material has a natural tendency to shrink as it is cooled in the mold and as it cools in the air after it exits from the mold. Such shrinkage may cause the sheet to be pulled excessively against the internal walls in the guide rails, resulting in excessive frictional drag when the sheet is advanced.